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Clusters in sedimentation equilibrium for an experimental hard-sphere-plus-dipolar Brownian colloidal system.

Newman HD, Yethiraj A - Sci Rep (2015)

Bottom Line: Care is taken to ensure that both the dimensionless gravitational energy, which is equivalent to a Peclet number Peg, and dipolar strength Λ are of order unity.In the presence of an external electric field, anisotropic chain-chain clusters form; this cluster formation manifests itself with the appearance of a plateau in the diffusion coefficient when the dimensionless dipolar strength Λ ~ 1.The structure and dynamics of this chain-chain cluster state is examined for a monodisperse system for two particle sizes.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Physical Oceanography, Memorial University, St. John's, NL, A1B 3X7, Canada.

ABSTRACT
In this work, we use structure and dynamics in sedimentation equilibrium, in the presence of gravity, to examine, via confocal microscopy, a Brownian colloidal system in the presence of an external electric field. The zero field equation of state (EOS) is hard sphere without any re-scaling of particle size, and the hydrodynamic corrections to the long-time self-diffusion coefficient are quantitatively consistent with the expected value for hard spheres. Care is taken to ensure that both the dimensionless gravitational energy, which is equivalent to a Peclet number Peg, and dipolar strength Λ are of order unity. In the presence of an external electric field, anisotropic chain-chain clusters form; this cluster formation manifests itself with the appearance of a plateau in the diffusion coefficient when the dimensionless dipolar strength Λ ~ 1. The structure and dynamics of this chain-chain cluster state is examined for a monodisperse system for two particle sizes.

No MeSH data available.


Effect of an electric field (E = 0 V/mm to E = 1666.7 V/mm) on sedimentation equilibrium.0.8 μm diameter PMMA colloids in 70:30 decalin/TCE in the presence of gravity. (a) Sedimentation profiles become progressively more extended as a function of the field strength. (b) The zero-field equation of state (field strengths are as in the legend in (a)) is consistent with hard spheres (the solid black line is the Carnahan-Starling equation of state), while the osmotic pressure is increasingly higher than the hard-sphere value with increasing field strength.
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f2: Effect of an electric field (E = 0 V/mm to E = 1666.7 V/mm) on sedimentation equilibrium.0.8 μm diameter PMMA colloids in 70:30 decalin/TCE in the presence of gravity. (a) Sedimentation profiles become progressively more extended as a function of the field strength. (b) The zero-field equation of state (field strengths are as in the legend in (a)) is consistent with hard spheres (the solid black line is the Carnahan-Starling equation of state), while the osmotic pressure is increasingly higher than the hard-sphere value with increasing field strength.

Mentions: Figure 2(a) shows the sedimentation profiles, Φ as a function of z, for all fields for the 0.8 μm-diameter colloids (mean volume fraction Φ = 0.017), obtained from the 3-dimensional image stacks. We see in Fig. 2(a) that the profiles are more extended as the field increases. This is expected due to the onset of the formation of string-like clusters of particles, which we already saw in Fig. 1. One can also calculate the equation of state by simultaneously obtaining the local volume fraction Φ(z) in thin slices dz at height z, parallel to and above the bottom substrate and the local osmotic pressure as a function of field strength is shown in Fig. 2(b)23. We can see that the zero field equation of state (EOS) for 0.8 μm PMMA particles shows excellent agreement with what is expected for a hard-sphere EOS from the Carnahan-Starling relation, and is in agreement with careful experiments in true bulk (non-microscopy) systems2425. This shows that our system is an excellent hard-sphere system, at least in the dilute regime which is of interest here. While the packing fraction range for these experiments is low, it has been seen in previous work that deviation from hard-sphere-like behaviour, for example due to a soft particle shell, manifests itself in a larger effective particle diameter even at low concentrations. For example, in the work of Li et al.23, which was a silica in water-DMSO colloidal system where the void phase was observed6, the EOS could only be fit by assuming an effective particle diameter that was 20% larger than the measured value.


Clusters in sedimentation equilibrium for an experimental hard-sphere-plus-dipolar Brownian colloidal system.

Newman HD, Yethiraj A - Sci Rep (2015)

Effect of an electric field (E = 0 V/mm to E = 1666.7 V/mm) on sedimentation equilibrium.0.8 μm diameter PMMA colloids in 70:30 decalin/TCE in the presence of gravity. (a) Sedimentation profiles become progressively more extended as a function of the field strength. (b) The zero-field equation of state (field strengths are as in the legend in (a)) is consistent with hard spheres (the solid black line is the Carnahan-Starling equation of state), while the osmotic pressure is increasingly higher than the hard-sphere value with increasing field strength.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4555105&req=5

f2: Effect of an electric field (E = 0 V/mm to E = 1666.7 V/mm) on sedimentation equilibrium.0.8 μm diameter PMMA colloids in 70:30 decalin/TCE in the presence of gravity. (a) Sedimentation profiles become progressively more extended as a function of the field strength. (b) The zero-field equation of state (field strengths are as in the legend in (a)) is consistent with hard spheres (the solid black line is the Carnahan-Starling equation of state), while the osmotic pressure is increasingly higher than the hard-sphere value with increasing field strength.
Mentions: Figure 2(a) shows the sedimentation profiles, Φ as a function of z, for all fields for the 0.8 μm-diameter colloids (mean volume fraction Φ = 0.017), obtained from the 3-dimensional image stacks. We see in Fig. 2(a) that the profiles are more extended as the field increases. This is expected due to the onset of the formation of string-like clusters of particles, which we already saw in Fig. 1. One can also calculate the equation of state by simultaneously obtaining the local volume fraction Φ(z) in thin slices dz at height z, parallel to and above the bottom substrate and the local osmotic pressure as a function of field strength is shown in Fig. 2(b)23. We can see that the zero field equation of state (EOS) for 0.8 μm PMMA particles shows excellent agreement with what is expected for a hard-sphere EOS from the Carnahan-Starling relation, and is in agreement with careful experiments in true bulk (non-microscopy) systems2425. This shows that our system is an excellent hard-sphere system, at least in the dilute regime which is of interest here. While the packing fraction range for these experiments is low, it has been seen in previous work that deviation from hard-sphere-like behaviour, for example due to a soft particle shell, manifests itself in a larger effective particle diameter even at low concentrations. For example, in the work of Li et al.23, which was a silica in water-DMSO colloidal system where the void phase was observed6, the EOS could only be fit by assuming an effective particle diameter that was 20% larger than the measured value.

Bottom Line: Care is taken to ensure that both the dimensionless gravitational energy, which is equivalent to a Peclet number Peg, and dipolar strength Λ are of order unity.In the presence of an external electric field, anisotropic chain-chain clusters form; this cluster formation manifests itself with the appearance of a plateau in the diffusion coefficient when the dimensionless dipolar strength Λ ~ 1.The structure and dynamics of this chain-chain cluster state is examined for a monodisperse system for two particle sizes.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Physical Oceanography, Memorial University, St. John's, NL, A1B 3X7, Canada.

ABSTRACT
In this work, we use structure and dynamics in sedimentation equilibrium, in the presence of gravity, to examine, via confocal microscopy, a Brownian colloidal system in the presence of an external electric field. The zero field equation of state (EOS) is hard sphere without any re-scaling of particle size, and the hydrodynamic corrections to the long-time self-diffusion coefficient are quantitatively consistent with the expected value for hard spheres. Care is taken to ensure that both the dimensionless gravitational energy, which is equivalent to a Peclet number Peg, and dipolar strength Λ are of order unity. In the presence of an external electric field, anisotropic chain-chain clusters form; this cluster formation manifests itself with the appearance of a plateau in the diffusion coefficient when the dimensionless dipolar strength Λ ~ 1. The structure and dynamics of this chain-chain cluster state is examined for a monodisperse system for two particle sizes.

No MeSH data available.